The present invention generally relates to a process for the manufacture of tubing made of cellulose hydrate with a coating on its inside and apparatus for carrying out the process, and more particularly to a process for the manufacture of tubular packaging sheaths made from cellulose hydrate having, on the inside, a uniformly thick, uninterrupted film coating of a natural or synthetic polymer which is virtually impermeable to water and water vapor, and to apparatus for carrying out the process.
The invention also relates to the use of the products manufactured by the processes according to the invention as a packaging sheath, in particular for pasty foodstuffs, more especially as a synthetic sausage skin for sausage intended to be boiled or cooked.
British Patent Specification No. 1,201,830 describes a process for the internal coating of tubular packaging sheaths made of cellulose hydrate, with an aqueous polymeric dispersion, in which the cellulose hydrate tubing is continuously passed horizontally through the hip of a vertical pair of nip rolls and is thereafter deflected into the vertical direction by means of a deflecting roller. The cavity of the portion of tubing issuing from the nip of the pair of nip rolls contains a certain amount of aqueous polymeric dispersion as a coating liquid. The vertically running tubing is exposed to heat above the level of the quantity of coating liquid enclosed in the cavity of the tubing.
In a process, described in U.S. Pat. No. 2,901,358, for the internal coating of cellulose hydrate tubing with an aqueous polymeric solution, the tubing to be coated, in flattened form, is first guided horizontally and is then, with partial wrap-round of the circumference of one roll of a horizontal pair of nip rolls, passed through the nip of this pair and thereafter guided vertically upwardly A certain amount of coating liquid is enclosed in the tubing cavity above the nip of the pair of rolls. The exposure to heat, in order to dry the layer on the inside of the tubing, takes place after the tubing is pressed flat in the nip of a pair of nip rolls.
German Offenlegungsschrift No. 2,801,038 discloses a process for the internal coating of tubing made of plastic or of a dense fabric which is continuously passed through the nip of a vertical pair of nip rolls. The tubing, in flattened form, is fed to the pair of rolls, in an upwardly-inclined direction, before entering the nip of the pair of nip rolls. The tubing first rests, in flattened form, partially against the circumference of the lower roll of the pair of rolls, whilst after issuing from the metering nip it rests, in flattened form, partially against the upper roll. After the flattened tubing has been lifted off the surface of the upper roll of the pair of rolls, it proceeds in an upwardly-inclined direction. The coating liquid is contained in the portion of tubing which, viewed in the direction of travel of the tubing, is upstream of the pair of nip rolls.
The known processes for the internal coating of cellulose hydrate tubing with aqueous polymeric dispersions all have the disadvantage that the cellulose hydrate tubes, which because of the high water content of the polymeric dispersions used are heavily moistened with water, tend on subsequent drying to undergo an undesirable change of dimension due to shrinkage.
They further all have the disadvantage that because of the use of aqueous polymeric dispersions having a relatively low content of dispersed polymer, the layer of aqueous polymeric dispersion on the inside of the tubing runs freely off this inside--since the low viscosity of the dispersion means that it flows easily. As a result, the polymeric film coating on the inside of the tubing, which is formed from this layer of dispersion after expulsion of the dispersing medium therefrom and fusion of the dispersed component by the action of heat, is non-uniform, and the amount of water which, in order to form the film coating on the inside of the tubing by exposing the tubing to heat, has to be expelled and transported outwardly through the tubing wall by diffusion is large in relation to the amount of the dispersed polymeric component of the dispersion.
The known processes accordingly are disadvantageous in energy terms, and furthermore lead to a non-uniform film coating on the inside of the tubing.